<style>
  .parent {
    display: flex;
    height: 200px;
    width: 1000px;
    background-color: pink;
  }

  .a {
    width: 100px;
    flex-grow: 1;
    background-color: skyblue;
  }

  .b {
    width: 100px;
    flex-grow: 2;
    background-color: yellow;
  }
</style>

<div class="parent">
  <div class="a"></div>
  <div class="b"></div>
</div>
